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f : firewire@lists.apple.com 2 January 2006 • 5:07PM -0500

DCL timestamps accuracy and position / packets
by Ananda Tallur

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Hi,

I have a little question concerning timestamps in DCL program,

The DCL program I use is for receiving frames from an imaging device,  
using DCAM protocol.
I have just added into the DCL program, a TimeStamp instruction, just  
after the receive packets :

Label
Receive packets * N
TimeStamp
UpdateDCL
CallProc
Jump
.....

It works very well, and I receive perfectly well timestamp, with the  
following format :
0000 SSSC CCCC CCCC CCCC 0000 0000 0000,
with S = seconds, from 0 to 7.
C = cycle count : unit = 1/8000 s (125 microsecs), from 0 to 7999

My question :
-> Knowing that I have many received packets for each frame, to what  
packet does the DCL timestamp correspond to ? To the first packet,  
the last packet, or none?
Is it possible to have a timestamp corresponding to the beginning of  
first receive packet (beginning of frame transmission) ?

-> Unit for cycle count is 125 microsecs. What is the accuracy of  
OHCI clock generating these cycles, compared to the computer system  
clock, for instance ?

I thank you very much for your answers,
Wishing a happy new year to firewire list readers,

Anand
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